This invention relates generally to a lean NOx catalyst system for treatment of diesel engine exhaust to reduce nitrogen oxide (NOx) emissions and more specifically to a metal doped oxide catalyst having high deNOx selectivity for NOx reduction and a method for preparing the metal or metal oxide doped oxide catalyst. The preferred oxide support material is xcex3-alumina.
Due to primarily federal regulations, engine manufacturers are being forced to reduce the amount of harmful compounds in the combustion exhaust, including a reduction in nitrogen oxides or NOx. To effectively reduce the NOx concentrations in the exhaust stream of lean burning engines including diesel and certain spark ignited engines, the use of selective catalytic reduction of NOx with a hydrocarbon in oxygen rich conditions (lean NOx catalyst systems) is actively being pursued.
Unfortunately, many catalysts or catalytic systems that have been utilized for lean burn engines to date suffer from low removal of NOx, inadequate catalyst durability, low thermal stability and a limited temperature window of operation. For example most commercially available lean NOx catalytic systems for lean burn engines only achieve less than 20% NOx reduction as a passive system and possibly up to 40% for active systems (i.e. when supplemental hydrocarbon reductant is introduced into the exhaust stream). In addition, most commercially available lean NOx catalytic systems also are subjected to sulfur poisoning, from the minimal amounts of sulfur found in many fuels and lubricants, resulting in low catalyst durability.
Such active catalytic systems involving lean NOx catalysts require a sufficient concentration of hydrocarbon (HC) species to be present. In other words, in combustion exhaust purification systems having an oxygen environment above 4% concentration, some type of reducing agent, usually a hydrocarbon compound such as diesel fuel, must be introduced into the exhaust in order to achieve acceptable reduction levels of NOx compounds. However, such introduction of fuel tends to reduce the overall fuel efficiency of the engine system.
Among metal oxide materials, alumina has been known as a durable material as well as a promising catalyst for lean-NOx reactions at high temperature. Various prior art disclosures have reported that alumina showed high deNOx performance vis-à-vis NO2, particularly when oxygenated hydrocarbons were present in the exhaust gas stream. However, exhaust gas streams from most diesel engines also include NO constituent that must be readily converted.
What is needed therefore, is a lean NOx catalyst that has good selectivity of both NO and N02 in order to achieve both an improvement in the reduction of NOx from the engine exhaust stream with minimal fuel efficiency impact on the overall engine system. The presently disclosed catalyst is directed to overcome the problems as set forth above.
In one aspect, the disclosed invention is a method of making lean NOx catalyst. The disclosed method comprises the steps of: preparing a substrate; preparing a xcex3-alumina catalyst support material using a sol-gel process; doping metal promoters into the xcex3-alumina support material; and applying the metal doped xcex3-alumina support material to the substrate. The preparation of the xcex3-alumina support material by the sol-gel process is closely controlled to thereby controlling the xcex3-alumina support material physical properties including average pore size, pore volume, surface area, acidity, and impurity level.
In another aspect, the disclosed invention is a method of making a xcex3-alumina lean NOx catalyst comprising the steps of: dissolving an aluminum based precursor in a complexing agent to form an aluminum based solution; gelating the aluminum based solution to yield aluminum hydroxide; washing the aluminum hydroxide with water or an organic solvent; aging the aluminum hydroxide; drying the aluminum hydroxide; calcinating the aluminum hydroxide to form xcex3-alumina catalyst support material. Subsequent steps involve doping metal promoters selected from the group consisting of indium, gallium, tin, silver, germanium, gold, nickel, cobalt, copper, iron, manganese, molybdenum, chromium, cerium, vanadium, oxides thereof, and combinations thereof into the xcex3-alumina catalyst support material; and applying the metal doped xcex3-alumina catalyst to a ceramic substrate.